The present invention relates to a device for reading magnetic information, which device has at least one read head comprising at least one magnetoresistive bar intended for generating data pulses which represent information read by the head.
Such reading devices are commonly used for reading information stored on hard disks for computers. These disks generally have a magnetosensitive surface which is to be scanned by the read head. A rotary movement is imparted to the disk, while the read head is coupled to an arm which imparts a radial movement to the head. The disk surface is divided into a multitude of sub-surfaces in which a local magnetic field exists whose sign represents an item of binary information.
The operation of customary reading devices is based on the fact that when a magnetoresistive bar is exposed to a magnetic field its resistance varies.
In the majority of existing magnetic information reading devices the magnetoresistive bar is biased either by means of a d.c. bias current of predetermined value which flows through said bar, in which case the resistance variation generates a voltage pulse, or by means of a d.c. bias voltage of predetermined value applied across said bar, in which case said resistance variation generates a current pulse.
In the two afore-mentioned cases the amplitude of the generated pulses, called data pulses will be higher as the predetermined value, is higher. Therefore, it may seem advantageous to choose the predetermined value as high as possible in order to obtain a signal-to-noise ratio which is as high as possible. However, the predetermined value also dictates the power to be dissipated by the magnetoresistive bar. Thus, a predetermined value which is too large may lead to premature wear of the magnetoresistive bar as a result of an excessive permanent dissipation owing to the Joule effect. Moreover, the high amplitude of the data pulses resulting from such a choice may cause the appearance of power pulses is too high to be dissipated by the bar without damage and may lead to the destruction of the latter. Thus, the choice of the predetermined value appears to be a matter of a compromise between, on the one hand, an optimum signal-to-noise ratio for the data pulses and, on the other hand, a minimization of the wear and the risk of breakdown of the magnetoresistive bar, i.e. a minimization of the power dissipated by said bar.
In the present state of the art various methods of selecting the predetermined value are used, which all make use of the value assumed by the resistance of the magnetoresistive bar when this bar is not exposed to any magnetic information. Indeed, this value, referred to as the quiescent resistance, is essential for calculating, on the one hand, the amplitude which the data pulses will have and, on the other hand, the power dissipated in the bar. Thus, the value of the quiescent resistance is considered to be indispensable for determining the best compromise in accordance with the principle expounded above.
The known reading devices therefore include a system for measuring the quiescent resistance, which is usually intricate and which requires a substantial area of silicon for its implementation in integrated form as well as a considerable time for carrying out the actual measurement. The measurement of the quiescent resistance thus requires additional cost both in the fabrication of the reading device and in use.
It is an object of the invention to solve these problems by providing a magnetic information reading device in which the biasing of the magnetoresistive bar does not require a prior measurement of the quiescent resistance.
To this end, according to the invention, a magnetic information reading device of the type defined in the opening paragraph is characterized in that it includes a control loop for controlling the power dissipated in the magnetoresistive bar, the control loop having a time constant which is large with respect to the duration of the data pulses.
In such a reading device it is no longer a question of measuring the quiescent resistance and of deriving from this an optimum predetermined value for a bias voltage or current. On the contrary, it suffices to choose an optimum value for the power to be dissipated by the magnetoresistive bar, the control loop automatically controlling the operating conditions of said bar in such a manner that it effectively dissipates this power. The fact that the time constant of the control loop is large with respect to the duration of the data pules, enables to avoid that the loop distorts these pulses by attempting to subject them to an inappropriate control. Thus, only the d.c. parameters which govern the biasing of the bar are controlled. The data pulses are not affected by the power control, as a result of which the information represented by them is free from any alteration.
In an embodiment of the invention includes regulating means for simultaneously regulating the value of a current flowing through the magnetoresistive bar and the value of a voltage present across said bar, the control loop being controlled by a control signal whose value is representative of a regulated value of the power which is to be dissipated by said bar.
This structure of the control loop enables the regulated value to be determined simply by the choice of the control signal which controls said loop via the regulating means.
In a special embodiment of the invention the control loop includes a current multiplier arranged to receive a first current representative of the control signal, a second current representative of the current flowing through the magnetoresistive bar, and a third current representative of the voltage present across said bar, which multiplier is intended for generating a fourth current having a value whose square is proportional to the quotient between the values of the second and third currents, multiplied by the value of the first current.
It will be demonstrated hereinafter that the current multiplier enables to generate a signal, formed by the fourth current, whose value is independent of the electrical parameters which govern the operation of the device and, particularly, of the bias current and voltage of the magnetoresistive bar. This signal, which is exclusively representative of the regulated value and the quiescent resistance, enables to easily control the regulating means.
In an advantageous embodiment of the invention, the regulating means comprise a comparator intended for supplying an output signal representative of the difference between the fourth current and the current flowing though the magnetoresistive bar, and a transistor, whose conduction is controlled by the output signal of the comparator, which transistor has its main current path arranged in series with the magnetoresistive bar.
This structure of the regulating means, which is advantageous because of its simplicity, enables the bias current and voltage to be controlled simultaneously solely by means of the fourth current.
In a variant of this embodiment, the control loop includes a capacitive element intended for storing the value of the output signal of the comparator, which capacitive element has a value which is large enough to achieve that the duration of the data pulses is negligible with respect to the time constant of the control loop thus obtained.
The storage of the output signal of the comparator enables to lock the settings obtained during controlling. Moreover, the time constant of the control loop in the present embodiment depends on the value of the capacitive element. Therefore, it suffices to influence this value in order to adjust said time constant.
In a preferred embodiment of the invention, the control signal being of a digital nature, the control loop includes a register intended for storing the value of the control signal, and a digital-to-analog converter intended for receiving the content of said register and for converting said content into an analog current forming the first current.
This preferred embodiment of the invention enables to program the regulated value digitally, which is particularly advantageous in applications in the field of hard disks for computers, in which information is conveyed in digital form.